1. Field of the Invention
The present invention relates to variable force, constant frequency shakers, and more particularly relates to tree shakers having adjustable weights which may be moved between opposed balanced positions which results in no shaking action when being rotated at a desired frequency and may be adjusted when rotating between the balanced position and to a plurality of unbalanced positions to provide variable shaking forces of a plurality of desired magnitudes.
2. Description of the Prior Art
Mobile tree shakers for harvesting fruit or nuts from trees or the like are well known in the art. Assignee's Peters U.S. Pat. No. 4,194,347 discloses a conventional mobile tree shaker; and Vallicella U.S. Pat. No. 3,964,244 discloses a mobile tree shaker having wings for collecting the fruit or nuts shaken from trees.
The above conventional tree shakers include a shaking head which is clamped to the trunk or limbs of a tree to have fruit or nuts shaken therefrom. The above prior art shakers include two unbalanced, non-adjustable counter rotating weights which are secured to shafts and apply a shaking force to the trunk or limbs of the tree as soon as the shafts start to rotate. Thus, the shafts must be stopped when the shaking head is being clamped on or removed from the tree to avoid damage to the bark of the tree. Therefore, the shafts must be accelerated from 0 speed to a desired maximum speed to shake the tree and then be returned to 0 speed before releasing the tree. This results in high torques, undesirable resonant frequencies, and wasted time each time a new tree or new limb is to be shook.
The undesirable resonant frequencies tend to damage the bark each time the tree is clamped or unclamped. When starting or stopping, the shaker may take one or two wild oscillations if the operator is unclamping or clamping as the resonance occurs which can cause sever barking of the tree. This damage is due to torsional and translational natural frequencies in the shaking system.
Damage to the fruit is also noted in the prior art shakers during the start and end of each low frequency, high amplitude burst. These low frequency high amplitude excursions result in substantial damage to undetached fruit by hitting limbs or other undetached fruit. Also, with a major portion of the fruit being detached in a period of one or two seconds, considerable damage results from fruit landing on fruit on conventional catch frames of prior art systems.